Wide band frequency modulator



Dec.14,1948. ERSHENK ET AL y 2,456,089v

WIDE lBAND FREQUENCY MODULATOR 2 Sheets-Sheet 1 Filed June 26, 1946 ATTORNEY Dec. 14, 1948. E. R. sHENK ET AI- WIDE BAND FR`I:"(;)UF.}NCYl MODULATOR Filed June 26, 1946 2 Sheets-Sheet 2 ,m f w .Y www u q my@ g ma/7 n. www wm im m PM VII ATTORNEY Patented Dec. 14, 1948 UNITED STAT-Es `Plvrrziur oFFlcE l "i 'y `v2,456,089

WIDE BAND lirmetwnrzoiz MoDULA'ron Eugene R. Shenk, Brooklyn, and Anthony Liguori; New York, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Appiieation ruhe 2,6, 1946, serial No. 679,339

This application concerns a new and improved frequency modulator, particularly adapted for use at low frequencies such as, for example, audio frequencies. v'

In some systems known in the art frequency modulation in the audio rangehas been achieved with the use of two oscillators in the low radio frequency range beating against each other. For example, in Fig. 2 of Whitaker U. S. Patent y2,289,- 157 a pair of oscillators o and `o' operating at frequencies separated by a small frequency band are used as described above for production of frequency modulated currents of low frequency. In this patented arrangement the oscillators o and o yoperate at slightly different radio frequencies. One of the oscillators o is modulated in frequency by currents or potentials derived by scanning and by other potentials. The oscillatory energy generated in the two loscillators are applied to separate grids in the converter tube 16 and the difference frequencyis taken from the output 'of the converter.` vThe difference beat note derived from the two oscillators is also modulated in frequency and is selected for sig'- naling purposes. Other systems of this type and systems which are more complicated areknown in the prior art.

As Will be seen by consideration'o'f'the System described above an oscillator and a mixer'stage, in addition to the modulated cscillatonareused in this type system. Moreover, in the two oscil-l lator systems as described above a small percentage frequency drift in eitherof the oscil4 lators results in a large Apercentage frequency drift in the audio frequency output l An object of my invention i's" provisionfof a wave length modulator adapted for timing modulation of carrierA currents in the relatively low frequency or audio range 'in which system very little frequency drift in the audio frequency output can take place.

Another obj ect of my invention is provision of a relatively inexpensive timing modulator supplying a relatively low frequency output with' little or no frequency drift. In using the termffrequency modulation applicant does not exclude modified frequency modulation 'or phase modulation or modulation having the characteristics of either frequency vor phase modulation.4 Moreover'although I have described'the carrier as being of low or audio frequency yobviously it may be of high or low frequency.

These main objects are accomplished in accordance with my invention by use of the current from al pentode tube integrated in a vcapacitor con-:-V

9 Claims.` (Cl. 179-1715) nected in a trigger or locking circuit having on condition of stability to which it tends to return. The arrangement is such that when the voltage across the capacitor reaches a critical magnitude the trigger circuit trips from itslstable condi?- tion and discharges the capacitor in a very short time. Ihenthe circuit returns to its stablecondition and the capacitor is allowed to recharge. Modulating current or voltage is applied to the pentode grid to control the magnitude of the current through the pentode and thereby produce frequency modulation of the trigger circuit by controlling the rate at which the capacitor is charged and discharged `and as-a consequence the repetition rate at which vthe trigger circuit is redtodischarge the capacitor.

In one application ofv my invention the output is taken from the trigger rcircuit described briefly above. In another applicationfthe'modulated trigger circuit controls a multivibratorwhich dividesby two. Both 'system require only one oscillator operating at an yaudio frequency rate.

In pentodes'tubes the currentv is constant for any one value 'of grid voltage irrespective of changes in the anode potential so that a linearl increase ordecrease in signal voltage' results in a linear increase or decrease in output frequency over very wide limits, for example, infan embodiment of my invention va 20 volt change in lsignal voltage results in a frequencyshift from 2,000 cycles per second to 12,000 cycles per second. Although it is believed that applicants invention as described hereinabove will be fully under-E stood by those skilled' in the art details of the.' same and the manner in which it is applied Will now be described. Inthis description reference will be made to the vattached drawings wherein Figs. 1 and 2 eachillustrate a different embodiment of my capacitor controlled triggercircuit with the means for controlling the charge of the capacitor in accordance with signals. In the em bodiment of Fig, 2 a multivibrator circuit isr 'conrnected to the modulated trigger circuit and the multivibrator is assumed to have a factor of;

division of two.

Fig. 3 illustrates by voltage waveforms the manner in which the modulators of Figs. 1v and y2v operate.

In Fig. 1 of the drawing, VI may be designated the modulator tube. Its grid 4 is connectedto the high potential side of a modulating signal input circuit thelowpotential side of which is grounded.

Thegrd.4"is'connected to the negative `terminal of va'biasing source C by resistor 6. The cathode is also connected to this lsource byl axed resistor cathode of tube V2 and to the anode of a diode tube V4 the cathode of which is grounded putting the tube V4 in shunt to capacitor Ci. The anode of tube V2 is connected toa ydirect current source by resistor I6 and tothecontrolgrid :of tube V3 by capacitor C2. The-:anode of Viris connected by resistor 22 to the grid of tube V2 and to ground so that the anode 'is suppliedtvith a direct current potential which is posi-true retative to its cathode because the cathode is connected tothenegativeterminal Aof .the said source. The grid. circuit. l.of .tu-beVZ includes a resistor 20 While .the grid circuitojf .tubeVLincludes-.a resistor .RL "The resistor .2.2 .-i-s shunted by Va capacitor ,2 B. I. Resistors y20and 22 are proportioned toprovide suitable. dir'ect currentbiasing` voltage furthe grid -of V2. `Capacitor 26 .constitutes a short .circuit yacross 26 for alternating currents.

In the embodiment illustrated. .the .tube-V2 is normally non-conductiveV since its grid is held negative with respect toground .by .the `potential drop produced .in resistor 2.2 due to currentilow in' the tube V3 which is normally conductive. If the point P is grounded tubes V2V and V3 are. in a. V4triggering l circuit operating as v.described above. V3 is normally conductive .because l.its cathode and grid are heldat essentially vthe saine potential -by the resistorRl. The arrangement is .suchthat when .current lows' in tube V3, which takes place when the .grid andc'athodeare of about .the same potential, thepotentiald'roplin. resistor 22 .is applied lto .the grid at tube'VZ tocut` this tube oi. Applicationot a less negative potential tothe controlgridof V2. Will. cause current to. flow in tubevVZ tocut off currentilow intheftube V3.. 'Ihispermitsthe v'chargeion the. condenser C2, built while tube V2 was non-conductive,` todecrease. While thischarge in the wnd'euserv C2 is decreasing vthe discl'iarge current `flowing through resistor iRlidevelops a negative potential n thecontrol grid of tube-V3 to .reduce current in this tube. When .the current is reducedV in tube V3 the potential dropin resistor 22 decreases to makethe controlgridin. .tube V2 relativelyrnore positive. y'.Ihelaction` isaccumula- V2. By the tripping action described above V3 is cut-off. Tube V2 on being tripped to the conductive state conducts for a time at least as long as the time required to discharge condenser CI. The time constant C2R| of the triggering circuit which determines the active time of the trigger circuit is slightly greater than the time required to :discharge condenser iC-I, but the diode V4' `supplies apath for the additional current to keep tube V2 conducting after capacitor CI is fully discharged.

The current flow from the tube Vl to condenser "fClis constant for any one value of grid signal volt tive and currentv is switched through =tube V2.

tube V2 negative with respect to -ground. The

grid. of tube V2. is normally held negative vwith respect to ,ground Vas described yhereinbefore. Assoon as the cathode .of4 tube-.VZbecomes negan tive enough,` that is,takes. on .a potential-.about like the.gridpotential.-.tube V2 vstarts conducting,

thereby discharging, condenser -GI :through tube4 Current flows fromI theplate age ontube Vil depsite the variations in plate volt-- age on theplate 'l of tube VI. Tube Vl, therefore, -suppliesra constantcurrent to Ci in the presence .of constant modulation potential irrespective of variations 'in' the potential at the point P. Any linear increase or decrease in signal voltage results in a linear increase or decrease in current flow ltocapacitor CIV. Since lcapacitor Cl charges to the. same Voltage value each time before discharging.the`.frequency produced .by the triggering circuit including tubes V2 and V3 is a function of the ...magnitude of the rcurrent flow through tube VI which in turnfollows the sig-nal potentials.

VWherevpulse typev outputis desired the embodiment -of Fig. 1 is used. Where sine wave output operationis required a multivibrator is connected tothe-plate of tube V2. The 4arrangement then is asillustrated` in Fig.. 2 wherein the tubes V5 and'VB .are in the multivibrator circuit. lThe operation of. the m-ultivibrators are `so well known inthe art thata detailed description thereof is thought-unnecessary here. The repetition rate of the multvilbrators depends, as is well known, on thfevcapacitors and resistors in the cross-coupled circuits. The anodes of the multivabrator tubes V5 `and V6 are supplied by potentials from the same source used to supply the triggering circuit through .resistor IE. To do this the ancdes oi tubes V5 rand V6 are connected by resistors 30 and` 32 to the anode of tube V2. Now as the anode potential of tube V2 rises and falls in accordance withA the ksignal modulations corresponding potential variations are applied to the anodes of tubes V5 and V6.in the multivibrator circuit to modulateits-frequency in accordance with the signals.

- In this operation then the multivibrator is synchronized by the pulses appearing at the anode of `tube V2 and at half the lmodulator frequency. A low pass filter is vconnected to the multivibrator andthis filter passes the fundamental frequency plus the modulation side bands. The filter output is sinusoidal in form.

vFigure 3 shows the voltage wave forms as they appear indifferent parts of the circuit. Line A shows the voltage across condenser CI, a linear charge and a short time discharge. The discharge pulses as they appear on the plate of V2 are shown in 'line B. The pulses synchronize the multivibratorat one-half theV modulator frequency and result .in Ithe square waves as shown in line C. These appear at the input t0 the low pass filter. The output `of the low pass filter is a sinusoidal wave as shown inline D.

What vis claimed is:

f l. A timing modulation system including in combination, `a trigger circuit including two electrocle 'structures with their electrodes crosscoupled by resistors and capacitors in such a manner that current normally ilovvs in one electrode structure and is cut oiin the other electrode structure, :a `capacitor connected to an electrode of one electrode structure to include the same in a path for said capacitor wherein current or potential is developed to control the charge in said capacitor to thereby control the potential on said electrode, a modulator tube having output electrodes in a second charge controlling circuit for said capacitor, and means for controlling the current through said modulator tube in accordance with signals.

2. A timing modulation system as recited in claim l wherein said modulator tube-is of the type wherein anode current depends primarily on grid potential rather than anode potential.

3. A timing modulation system including in combination, a trigger circuit including two tube structures with their electrodes cross-coupled by resistors and capacitors in such a manner that current normally flows in one structure and is cut off in the other structure, a capacitor connected to an electrode of said other structure to include the same in a discharge path for said capacitor and to control the potential on said electrode, a modulator tube having output electrodes in a charging circuit for said capacitor, and means for controlling the current through said modulator tube in accordance with signals.

4. A timing modulation system as recited in claim 3 wherein said modulator tube is of the type wherein anode current depends on grid potential rather than anode potential.

5. A timing modulation system including in combination, a trigger circuit including two tu-bes with their electrodes cross-coupled by resistors and capacitors in such a manner that current normally flows in one tube and is cut off in the other tube, a capacitor connected to an electrode oi one tube to include the same in a path for said capacitor wherein current or potential is developed to control the charge in said capacitor to thereby control the potential on said electrode, a modulator tube having output electrodes in a second charge controlling circuit for said capacitor, a multivibrator coupled to an electrode in one of said rst named tubes, and means for controlling the current through said modulator tube in accordance with signals.

6. A timing modulation system as recited in claim 5 wherein said modulator tube is of the type wherein anode current depends primarily on grid potential rather than anode potential.

7. A timing modulation system including in combination a trigger circuit including two tubes with their electrodes cross-coupled by resistors and capacitors in such a manner that current normally flows in one tube and is cut off in the other tube, a capacitor connected with one tube electrode to control the potential thereon, a modulator` tube having input electrodes excited by signals and having output electrodes and a coupling between the output electrodes oi said modulator tube and said capacitor.

8. A timing modulation system including in combination, a trigger circuit including two tubes with their electrodes cross-coupled by resistors and capacitors in such a manner that current normally flows in one tube and is cut ofi in the other tube, a capacitor connected with an electrode of said other tube to control the potential thereon and thereby control its state of conduction, and a modulator tube excited by signals and connected with said capacitor for controlling the rate of charge thereof in accordance with signals, the arrangement being such that when said condenser is charged the potential on said one tube electrode trips said trigger circuit so that current ilows in said other tube to discharge said capacitor.

9. A timing modulation system including in combination, a trigger circuit having a pair of electrode structures veach provided with an anode, a grid and a cathode, impedance elements crosscoupling the anodes and grids of said structures in such manner that current normally iiows in one electrode structure and is cut-ofi in the other electrode structure, a condenser shunted by a rectier connected between the cathode of the normally cut-off electrode structure and ground, a pentode modulator tube having an anode and a grid, a connection from the anode of said modulator tube to the cathode of said normally cut-oi electrode structure, and a connection from the grid of said modulator tube to a source oi modulating signals.

EUGENE R. SHENK. ANTHONY LIGUORI.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,193,850 Andrieu et al Mar. 19, 1940 2,394,427 Clark et al Feb. 5, 1946 2,418,268 Lamson Apr. 1, 1947 

